1. Technical Field
This invention relates to infrared windows and, more particularly, to fabrication of composite infrared windows by direct bonding.
2. Discussion
Conventional infrared (IR) windows for airborne applications lack high-speed sand and rain impact durability and, as a result, experience cumulative surface and subsurface damage which results in large optical transmittance loss. Repair or replacement of the IR window is expensive and time consuming.
Conventional IR windows rely primarily on two approaches to harden an outer surface thereof against high-speed rain and sand impact. In a first approach, a protective coating, for example boron phosphide (BP) or gallium phosphide (GaP), is deposited on top of an IR substrate. Although the protective coating substantially improves the durability of the IR window, cumulative damage still occurs both in the protective coating as well as in the IR substrate. Repair of IR windows fabricated according to the first approach is expensive and time consuming due to the difficulty of removing the protective coating completely and the need to repolish and recoat the IR substrate.
According to a second approach, a protective layer is bonded to an IR substrate using an adhesive to form the IR window. For example, a chalocogenide glass bonds a diamond film to a zinc sulfide (ZnS) substrate. Alternately, an organic polymer adhesive bonds a diamond film to a germanium (Ge) substrate. However, the intermediate bonding layer must satisfy stringent optical, thermal and mechanical requirements. Furthermore, it is difficult to remove the bonding material completely when replacement of the protective layer is required due to high-speed rain and sand impact. In addition, chalcogenide glasses are extremely toxic and difficult to handle during fabrication.
Therefore, an IR window which provides improved high-speed sand and rain impact durability and which can be repaired easily and at low cost would be desirable.